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Embodiments of the invention are directed to modified resin systems for
use in liquid resin infusion (LRI) processes, variations of LRI processes
and other suitable processes. In one embodiment, the modified resin
system includes a novel combination of at least one base resin, an amount
of particles within a predetermined range and an amount of thermoplastic
material within a predetermined range wherein, when combined, the
modified resin system has an average viscosity below a threshold average
viscosity at a specific temperature and a high level of toughness. The
modified resin system may additionally include a curing agent and other
suitable components. The modified resin system has been experimentally
shown to exhibit a unique, controllable and constant morphology which may
be at least partially responsible for imparting a required toughness and
damage resistance to a finished composite without adversely impacting
properties such as viscosity, potlife, cure temperature, glass transition
temperature or tensile modulus of the modified resin system.

1. A formulation, comprising: at least one base resin; an amount of
particles within a predetermined range in a carrier resin; and an amount
of thermoplastic material within a predetermined range wherein the base
resin, the particles and the thermoplastic material are combined to form
a modified resin system, the modified resin having an average viscosity
below a threshold average viscosity within a predetermined temperature
range.

3. The formulation of claim 1 wherein the base resin is one of epoxy,
bismaleimide, cyanate ester or a combination thereof.

4. The formulation of claim 3 wherein the base resin comprises a
combination of epoxies including at least one di- and tri-epoxy and at
least one tetra-epoxy.

5. The formulation of claim 1 wherein the particles are one of
functionalized core-shell rubber particles or hollow particles.

6. The formulation of claim 1 wherein the particles are one of
non-functionalized core-shell rubber particles or hollow particles.

7. The formulation of claim 5 wherein the functionalized core-shell
rubber particles comprise a core material which is one of
polybutadiene-styrene, polybutadiene or a combination thereof, and a
shell material which is one of silica, polymerized monomers of acrylic
acid derivatives containing the acryl group including acrylic and
poly(methyl methacrylate) or a combination thereof.

8. The formulation of claim 6 wherein the non-functionalized core-shell
rubber particles comprise a core material which is one of
polybutadiene-styrene, polybutadiene or a combination thereof, and a
shell material which is one of silica, polymerized monomers of acrylic
acid derivatives containing the acryl group including acrylic and
poly(methyl methacrylate) or a combination thereof.

9. The formulation of claim 1 wherein, in a cured condition, the
particles are substantially uniformly dispersed throughout the modified
resin system.

11. The formulation of claim 1 wherein the amount of thermoplastic
material is below approximately 30% net weight of the modified resin
system.

12. The formulation of claim 1 wherein, in a cured condition, at least
the thermoplastic material is phase separated from the base resin.

13. The formulation of claim 1 wherein the threshold average viscosity is
less than 5 Poise at a temperature of less than 180.degree. C.

14. A composite article, comprising: a structure having a predetermined
shape, the structure having a plurality of layers of a fiber-based
fabric, the structure having a targeted composite toughness within a
predetermined range, wherein the toughness is at least partially imparted
by a modified resin system during a process, the modified resin system
including: (i) at least one base resin; (ii) an amount of particles
within a predetermined range in a carrier resin; and (iii) an amount of
thermoplastic material within a predetermined range wherein the base
resin, the particles and the thermoplastic material are combined to form
the modified resin system, the modified resin having a average viscosity
below a threshold average viscosity within a predetermined temperature
range.

15. The composite article of claim 14 wherein the structure exhibits a
high level of microcrack resistance.

16. The composite article of claim 14 wherein the process is a liquid
resin infusion manufacturing process, a prepreg manufacturing process or
a resin film infusion process.

17. A formulation, comprising: a base resin comprising at least one
epoxy; a curing agent; an amount of thermoplastic material; and an amount
of core-shell particles wherein the base resin, the curing agent, the
thermoplastic material and the particles are combined to form the
modified resin system, the modified resin having an amount of
thermoplastic material of less than 30% net weight of the total weight of
the modified resin system.

18. The formulation of claim 17 wherein, in a cured or vitrified
condition, the thermoplastic material separates into aggregate domains
from the base resin, each aggregate domain having an island-like
morphology, the morphology evolving (i) during the later stages of a ramp
to dwell temperature or (ii) after a ramp to dwell has been completed
during the cure cycle.

19. A manufacturing process, comprising: preparing a preform; laying the
preform within a mold; heating the mold to a predetermined temperature;
and injecting a resin wherein the resin is a modified resin, the modified
resin system comprising a combination of: (i) at least one base resin;
(ii) a curing agent; (iii) an amount of particles within a predetermined
range in a carrier resin; and (iv) an amount of thermoplastic material
within a predetermined range wherein the amount of thermoplastic material
of the modified resin is less than 30% net weight of the total weight of
the modified resin system.

20. The manufacturing process of claim 19 wherein the predetermined
temperature of the mold is less than 180.degree. C.

21. The manufacturing process of claim 19, further comprising, ramping a
temperature of the mold to 180.degree. C. at a rate of up to 10.degree.
C. per minute.

22. The manufacturing process of claim 19 wherein, when the mold reaches
180.degree. C., the temperature is held for between 30 minutes and 150
minutes.

[0002] Liquid resin infusion (LRI) is a process used to manufacture
fiber-reinforced composite structures and components for use in a range
of different industries including the aerospace, transport, electronics,
and building and leisure industries. The general concept in LRI
technology involves infusing resins into a fiber reinforcement, fabric or
a pre-shaped fibrous reinforcement ("preform") by placing the material or
preform into a mold (two-component mold or single-sided mold) and then
injecting resin under high pressure (or ambient pressure) into the mold
cavity or vacuum bag sealed single-sided mold. The resin infuses into the
material or preform resulting in a fiber-reinforced composite structure.
LRI technology is especially useful in manufacturing complex-shaped
structures which are otherwise difficult to manufacture using
conventional technologies. Variation of liquid resin infusion processes
include, but are not limited to, Resin Infusion with Flexible Tooling
(RIFT), Constant Pressure Infusion (CPI), Bulk Resin Infusion (BRI),
Controlled Atmospheric Pressure Resin Infusion (CAPRI), Resin Transfer
Molding (RTM), Seemann Composites Resin Infusion Molding Process
(SCRIMP), Vacuum-assisted Resin Infusion (VARA) and Vacuum-assisted Resin
Transfer Molding (VARTM).

[0003] Most resin infusion systems are inherently brittle, and the
viscosity levels necessary to achieve the injection process preclude the
use of toughening agents. Said differently, the properties of toughness
and low viscosity are typically mutually exclusive in conventional resin
infusion systems. In prepregs, high levels of toughness are generally
achieved through the addition of about ten percent (10%) to about thirty
percent (30%) by weight of a thermoplastic toughener to the base resin.
However, addition of such tougheners to LRI systems generally results in
an unacceptable increase in the viscosity of the resin and/or reduction
in resistance of the cured material to solvents. In the specific case of
particulate toughener, there may be additional filtering issues in the
textile. These limitations render the addition of tougheners
conventionally added in prepregs generally unsuitable in conventional LRI
applications.

[0004] One technology to toughen fiber-reinforced composite structures
manufactured by LRI technologies is to integrate the toughener into the
preform itself. For example, a soluble toughening fiber may be directly
woven into the preform thereby eliminating the need to add toughener into
the resin which otherwise would increase the viscosity of the resin
(rendering it unsuitable for resin infusion). Another example is the use
of soluble or insoluble veils comprising of toughener used as an
interleaf with the reinforcement of the preform. However, in either of
these methods, the manufacturing process may be more complicated and
costly, in addition to increasing the risk of hot/wet performance
knock-downs and solvent sensitivity with a polymer based insoluble
interleaf. Another technology is the addition of particles to the resin.
The amount of particles required to reach a suitable toughness threshold,
however, is often high resulting in a viscous resin requiring a very
narrow process window that is generally unfavorable for LRI.

SUMMARY OF INVENTION

[0005] A formulation, comprising: (i) at least one base resin; (ii) an
amount of particles within a predetermined range in a carrier resin; and
(iii) an amount of thermoplastic material within a predetermined range
wherein the base resin, the particles and the thermoplastic material are
combined to form a modified resin system, the modified resin having an
average viscosity below a threshold average viscosity within a
predetermined temperature range is herein disclosed. The formulation may
further comprise a curing agent. The curing agent may be an aniline-based
amine compound. The base resin may be one of epoxy, bismaleimide, cyanate
ester or a combination thereof. The base resin may be a combination of
epoxies including at least one di-, tri- or tetra-epoxy. The particles
may be one of chemically functionalized or chemically non-functionalized
core-shell rubber particles or hollow particles. A material comprising
the core may be one of polybutadiene-styrene, polybutadiene or a
combination thereof, and a material comprising the shell may be one of
silica, polymerized monomers of acrylic acid derivatives containing the
acryl group including acrylic and poly(methyl methacrylate) or a
combination thereof. In a cured condition, the particles may be
substantially uniformly dispersed throughout the modified resin system.
The thermoplastic material may be one of phenoxy-based polymers,
poly(ether sulfone) polymers, poly(ether ether sulfones), poly(methyl
methacrylate) polymers, carboxylterminated butadiene acrylonitrile
polymers, copolymers thereof, or combinations thereof. The formulation
wherein the amount of thermoplastic material is below approximately 30%
net weight, preferably below 7%, of the modified resin system. In a cured
condition, at least the thermoplastic material phase may separate from
the base resin. More particularly, the thermoplastic material phase may
separate into aggregate domains from the base resin, each aggregate
domain having an island-like morphology. The morphology in a cured
article may evolve: (i) during the later stages of a ramp to dwell
temperature; or (ii) after a ramp to dwell has been completed during the
cure cycle. The amount of particles and the amount of thermoplastic
material may be combined in a 1 to 0.56 ratio. The threshold average
viscosity may be less than 5 Poise at a temperature of less than
180.degree. C., more narrowly between 80.degree. C. and 130.degree. C.

[0006] A composite article, comprising: a structure having a predetermined
shape, the structure having a plurality of layers of a fiber-based
fabric, the structure having a targeted composite toughness within a
predetermined range, wherein the toughness is at least partially imparted
by a modified resin system during a process, the modified resin system
including: (i) at least one base resin; (ii) an amount of particles
within a predetermined range in a carrier resin; and (iii) an amount of
thermoplastic material within a predetermined range wherein the base
resin, the particles and the thermoplastic material are combined to form
the modified resin system, the modified resin having a average viscosity
below a threshold average viscosity within a predetermined temperature
range is herein disclosed.

[0007] The modified resin system may further include a curing agent, the
curing agent comprising an aniline-based amine compound. The base resin
may be one of epoxy, bismaleimide, cyanate ester or a combination
thereof. The base resin may include a combination of epoxies including at
least one di-, tri- or tetra-epoxy. The particles may be one of
core-shell rubber (CSR) particles or hollow particles wherein, when the
particles are CSR particles, a material comprising the core is one of
polybutadiene-styrene, polybutadiene or a combination thereof, and a
material comprising the shell is one of silica, polymerized monomers of
acrylic acid derivatives containing the acryl group including acrylic and
poly(methyl methacrylate) or a combination thereof. In a cured condition,
the particles may be substantially uniformly dispersed throughout the
modified resin system. The thermoplastic material may be one of
phenoxy-based polymers, poly(ether sulfone) polymers, poly(ether ether
sulfones), polymerized monomers of acrylic acid derivatives containing
the acryl group including acrylic and poly(methyl methacrylate) polymers,
carboxylterminated butadiene acrylonitrile polymers, copolymers thereof,
or combinations thereof. The amount of thermoplastic material is below
approximately 30% net weight, preferably below 7% net weight, of the
modified resin system. With the base resin in a partially cured or
gel-like state, the thermoplastic material may separate into aggregate
domains from the base resin, each aggregate domain having an island-like
morphology. The amount of particles and the amount of thermoplastic
material may be combined in a 1 to 0.56 ratio. The structure may exhibit
a high level of microcrack resistance. The threshold average viscosity
may be less than 5 Poise at a temperature of less than 180.degree. C.,
more narrowly between 80.degree. C. to 130.degree. C. The fiber-based
fabric may be comprised of reinforcing fibers of a material selected from
the group consisting of organic polymer, inorganic polymer, carbon,
glass, inorganic oxide, carbide, ceramic, metal or a combination thereof.
The process may be a liquid resin infusion manufacturing process, a
prepreg manufacturing process or a resin film infusion process.

[0008] A formulation, comprising: (i) a base resin comprising at least one
epoxy; (ii) a curing agent; (iii) an amount of thermoplastic material;
and (iv) an amount of core-shell particles wherein the base resin, the
curing agent, the thermoplastic material and the particles are combined
to form the modified resin system, the modified resin having an amount of
thermoplastic material of less 30% net weight, preferably less than 7%
net weight, of the total weight of the modified resin system is herein
disclosed.

[0009] With the base resin in a partially cured or gel-like state, the
thermoplastic material phase may separate into aggregate domains from the
base resin. The amount of particles and the amount of thermoplastic
material may be combined in a 1 to 0.56 ratio. With the base resin in a
partially cured, gel-like, cured or vitrified state the particles are
substantially uniformly dispersed throughout the modified resin system.
The modified resin system may have an average viscosity of less than 5
Poise at a temperature of less than 180.degree. C., more narrowly between
80.degree. C. and 130.degree. C. With the base resin in a cured or
vitrified condition, the thermoplastic material may separate into
aggregate domains from the base resin, each aggregate domain having an
island-like morphology. The morphology in a cured article may evolve (i)
during the later stages of a ramp to dwell temperature or (ii) after a
ramp to dwell has been completed during the cure cycle.

[0010] A manufacturing process, comprising: (i) preparing a preform; (ii)
laying the preform within a mold; (iii) heating the mold to a
predetermined temperature; and (iv) injecting a resin wherein the resin
is a modified resin, the modified resin system comprising a combination
of (i) at least one base resin; (ii) a curing agent; (iii) an amount of
particles within a predetermined range in a carrier resin; and (iv) an
amount of thermoplastic material within a predetermined range wherein the
amount of thermoplastic material of the modified resin is less than 30%
net weight, preferably less than 7% net weight, of the total weight of
the modified resin system is herein disclosed.

[0011] The predetermined temperature of the mold may be 110.degree. C. The
manufacturing process may further comprise ramping a temperature of the
mold to 180.degree. C. at a rate of less than 10.degree. C. per minute,
more narrowly, less than 5.degree. C. per minute. The manufacturing
process wherein, when the mold reaches 180.degree. C., the temperature is
held for between 90 minutes and 150 minutes. The preform may be sealed
within the mold by at least a vacuum bag. An average viscosity of the
modified resin system may be less than 5 Poise at a temperature range of
less than 180.degree. C., more narrowly between 80.degree. C. and
130.degree. C. The preform may be comprised of plurality of layers of
fiber-based fabric. The fiber-based fabric may have a structure
comprising one of woven fabrics, multi-warp knitted fabrics, non-crimp
fabrics, unidirectional fabrics, braided socks and fabrics, narrow
fabrics and tapes or fully-fashioned knit fabrics. The fiber-based fabric
may be comprised of reinforcing fibers of a material such as organic
polymer, inorganic polymer, carbon, glass, inorganic oxide, carbide,
ceramic, metal or a combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 is a schematic illustrating conventional toughened resin
systems and the modified resin system according to an embodiment of the
invention.

[0013] FIG. 2 is a chart showing the relationship between viscosity and
toughness for a thermoplastic material in a base resin, core-shell
particles a base resin, and a combination of thermoplastic material and
core-shell particles in a base resin according to an embodiment of the
invention.

[0014] FIG. 3A is an optical micrograph of thermally-evolved thermoplastic
domains and core-shell rubber particle regions at increasing
concentration, but at a constant ratio of core-shell
particles:thermoplastic toughener in a modified resin system according to
an embodiment of the invention.

[0015] FIG. 3B is an optical microscopy evaluation of the thermally
evolved thermoplastic domains and CSR particle regions in the proposed
invention demonstrating the influence of CSR concentration on the
dimensions of the witnessed morphology.

[0016] FIG. 4 is a scanning electron microscopy (SEM) image of the island
like morphology and core shell particles witnessed in a cured and
modified resin system with respect to damage resistance mechanisms
according to an embodiment of the invention.

[0017] FIG. 5 is a graph comparing the fracture toughness of a modified
resin system according to embodiments of the invention to the fracture
toughness of other resin systems.

[0018] FIG. 6 Describes the evolution of the morphology as represented by
an embodiment of the current invention as a function of temperature or
vitrfication onset in the base resin comprising the proposed invention.

[0019] FIGS. 7A and 7B are Transmission electron Microscopy images of the
island like morphology and core shell particles witnessed in a cured and
modified resin system with respect to damage resistance mechanisms
according to an embodiment of the invention.

[0020] FIG. 8 Is an expanded SEM image detailing the growth rings in the
thermoplastic domains present in the proposed invention.

[0021] FIG. 9 illustrates a schematic of the generalized morphology of a
modified resin system according to embodiments of the invention

[0022] FIG. 10 Illustrates a representative LRI system having a fabric
perform thereon.

[0023] FIG. 11 is a chart comparing CSAI values of the modified resin
system according to embodiments of the invention to the CSAI values for
other resin systems.

DETAILED DESCRIPTION

[0024] The following detailed description is of the best currently
contemplated modes of carrying out the invention. The description is not
to be taken in a limiting sense, but is made merely for the purpose of
illustrating the general principles of the invention.

[0025] Embodiments of the invention are directed to modified resin systems
for use in resin infusion (RI) processes, variations of LRI processes and
other suitable processes such as prepreg processes. In one embodiment,
the modified resin system includes a novel combination of at least one
base resin, an amount of particles within a predetermined range and an
amount of thermoplastic material within a predetermined range wherein,
when combined, the modified resin system has an average viscosity below a
threshold average viscosity within a specific temperature range and a
high level of toughness. The modified resin system may additionally
include a curing agent and other suitable components. The modified resin
system has been experimentally shown to exhibit a unique, controllable
and constant morphology which is substantially or completely responsible
for imparting a required toughness and damage resistance to a finished
composite article without adversely impacting resin properties such as
viscosity, potlife, cure temperature, glass transition temperature or
tensile modulus of the modified resin system.

[0026] According to embodiments of the invention, a combination of at
least one base resin, an amount of particles within a predetermined range
and an amount of thermoplastic material within a predetermined range, in
addition to other components, may be combined in a "one pot" formulation
to generate a modified resin system which can be used in RI/LRI processes
or prepreg processes. The modified resin system as formulated according
to embodiments of the invention was discovered to have an unexpectedly
low viscosity, low reactivity, a high level of toughness (G.sub.1C),
among other characteristics, when subjected to numerous experimental
tests. It is anticipated that the modified resin may also be used in
variations of liquid resin infusion processes including, but not limited
to, Resin Infusion with Flexible Tooling (RIFT), Constant Pressure
Infusion (CPI), Bulk Resin Infusion (BRI), Controlled Atmospheric
Pressure Resin Infusion (CAPRI), Resin Transfer Molding (RTM), Seemann
Composites Resin infusion Molding Process (SCRIMP), Vacuum-assisted Resin
Infusion (VARI), Resin Transfer Injection (RTI) and Vacuum-assisted Resin
Transfer Molding (VARTM) as well as other processes used to manufacture
composite articles.

[0027] FIG. 1 is a schematic illustrating conventional resin systems and
the modified resin system according to an embodiment of the invention.
Numerical reference 102 represents an unmodified neat epoxy which may be
used in composite manufacturing processes. An unmodified epoxy resin
system is generally known to be unsuitable in the manufacture of high
toughness composite articles without resorting to the use of a secondary,
insoluble toughening article such as a hot-melt adhesive web, e.g.,
SPUNFAB.RTM. veil. Numerical reference 104 represents a modified epoxy
system having core-shell rubber (CSR) particles therein to impart a
toughening characteristic. Typically, modified epoxy systems of this type
are known to exhibit high toughness values which often do not translate
into equivalent composite performance. Numerical reference 106 represents
another modified epoxy system having a thermoplastic therein. This
modified epoxy system is known to have an average viscosity which is
outside of acceptable processing windows for LRI applications.

[0028] Numerical reference 108 represents a modified resin system
according to embodiments of the invention which is characterized by
having a suitable average viscosity for LRI (e.g., less than 5 Poise)
without sacrificing performance in the resin or composite, specifically
related to toughness properties. Modified resin system 108 includes at
least one base resin, an amount of particles within a predetermined range
and an amount of thermoplastic material within a predetermined range in a
novel combination which makes it suitable for LRI processes, prepreg
processes and other like processes. In FIG. 1, the base resin is an epoxy
resin or combination of epoxy resins; however, embodiments of the
invention are not limited to epoxy resins.

[0029] In the context of this application, a "resin" is a synthetic
polymer compound which begins in a viscous state and hardens with
treatment. Resins are used as a structural matrix material in the
manufacture of adhesives and composites and are often reinforced with
fibers (e.g., glass, Kevlar, Boron and Carbon). In some embodiments, the
base resin may be any one of epoxy, bismaleimide, benzoxazine, cyanate
ester, vinyl ester, polyisocyanurates, bismalimide, cyanate ester,
phenolic resin or any combination thereof in addition to other suitable
resins. In some embodiments, the base resin is an epoxy resin or a
combination of epoxy resins. The epoxy resin may be a tetra-, tri-,
di-epoxy or combinations of tetra-, tri- and/or di-epoxies. Exemplary
tri-epoxies include triglycidyl p-aminophenol (MY-0510 available from
Huntsman Advanced Materials, Inc.) and ARALDITE.RTM. (MY-0600 available
from Huntsman Advanced Materials, Inc.). An exemplary tetra-epoxy is
tetraglycidyl diaminodiphenyl methane (MY-721 available from Huntsman
Advanced Materials, Inc.). Other suitable epoxy resins include bisphenol
F epoxy (PY-306 available from Ciba Geigy).

[0030] In the context of this application, a "particle" is a polymer-based
material having a core-shell or hollow morphology. Core-shell rubber
(CSR) particles have the characteristic of having a core comprising of a
rubbery material surrounded by an outer shell of glassy material. CSR
particles are used as toughening agents when combined with polymeric
matrices, e.g., epoxy resins. In some embodiments, the particles may be
any commercially available chemically functionalized or chemically non
functionalized CSR particles having a core material of
polybutadiene-styrene or polybutadiene and having a shell material of
silica or polymerized monomers of acrylic acid derivatives containing the
acryl group including acrylic and poly(methyl methacrylate). The CSR
particles may be supplied in a carrier resin such as tetraglycidyl
diaminodiphenyl methane (i.e., MY-721) and may have a diameter of between
about fifty (50) nanometers (nm) and about eight hundred (800) nm, in one
embodiment, about one-hundred (100) nm. Examples of commercially
available CSR particles include, but are not limited to, the Paraloid
series of materials (available from Rohm and Haas), MX411
(polybutadiene-styrene/acrylic) and MX416 (polybutadiene/acrylic) (both
are dispersions in Huntsman MY721 epoxy resin and are available from
Kaneka Corp.); however, any particle exhibiting the CSR or hollow
structure as described above may be used in the modified resin systems
according to embodiments of the invention.

[0031] Core-shell particles have been evidenced to toughen LRI systems via
a cavitation mechanism in addition to crack pinning or "tear out"
mechanisms. In a cavitation mechanism, the rubbery cores of the CSR
particles yield under the stress concentrations at a crack tip, resulting
in dissipation of energy from the crack front and the formation of voids
in the core material.

[0032] In the context of this application, a "thermoplastic" is a polymer
that is elastic and flexible above a glass transition temperature
(T.sub.g). In some embodiments, the thermoplastic Material comprises one
of phenoxy-based polymers, poly(ether sulfone) (PES) polymers, poly(ether
ether sulfones), polymerized monomers of acrylic acid derivatives
containing the acryl group including acrylic and poly(methyl
methacrylate) (PMMA) polymers, carboxyl terminated butadiene
acrylonitrile (CTBN) polymers, copolymers thereof; or combinations
thereof. Representative thermoplastics include, but are not limited to,
KM180 (available from Cytec Industries. Inc.), 5003P (available from
Sumitomo Corp.), PKHB (InChemRes); however, any thermoplastic or other
suitable material (e.g., Nanostrength X, available from Arkema, Inc.)
exhibiting a thermally driven phase separation from a base resin, more
particularly, exhibiting aggregate domains, or an "island-like"
morphology (explained in more detail below), may be used in the modified
resin systems according to embodiments of the invention.

[0033] An example of a typical mechanism for thermoplastic toughening of
composite or resin matrices is crack pinning. Indications of crack
pinning mechanisms include tailing behind thermoplastic domains or
apparent plastic deformation around such thermoplastic zones originating
from a divergent crack front around a thermoplastic rich region and
subsequent convergence of the split crack fronts. Another example of a
typical toughening mechanism is that of ductile tearing which can be
described as a localized plastic deformation upon application of a stress
to the material.

[0034] A "curing agent" is a substance or mixture of substances added to a
polymer composition (e.g., resin) to promote or control the curing
reaction. Addition of curing agent functions to toughen and harden a
polymer material by cross-linking of polymer chains. Representative
curing agents include, but are not limited to, methylenebis (3-chloro-2,6
diethylaniline) (MCDEA), 3,3'-diaminodiphenyl sulfone (3,3'-DDS),
4,4'-diaminodiphenyl sulfone (4,4'-DDS), dicyandiamide (DICY),
N-methyl-diethanolamine (MDEA) and
4,4'-methylene-bis-(2-isopropyl-6-methyl-aniline) (MMIPA).

[0035] According to embodiments of the invention, the modified resin
system may include a thermoplastic which is 7% or less net weight of the
modified resin system combined with an amount of CSR particles in a 1 to
0.56 ratio of thermoplastic to CSR particles. In one embodiment, the base
resin may be a combination of di-, tetra- and tri-epoxies such as PY-306,
MY-0500 and/or MY-0600). In one embodiment, the thermoplastic material
may be 5003P and the CSR particles may be MX411 (in MY-721) or MX416 (in
MY-721) one-hundred (100) nm particles. A curing agent, such as MCDEA may
be added to the "one pot" resin system to make the resin system curable
when heat and/or pressure is/are applied thereto.

[0036] The formulation of the present invention comprises at least one
base resin; an amount of particles within a predetermined range in a
carrier resin; and an amount of thermoplastic material within a
predetermined range wherein the base resin, the particles and the
thermoplastic material are combined to form a modified resin system, the
modified resin having an average viscosity below a threshold average
viscosity within a predetermined temperature range. The threshold average
viscosity of the formulation is less than 5 Poise at a temperature of
less than 180.degree. C. and preferably at a temperature of between
80.degree. C. and 130.degree. C.

[0037] When the formulation is in a cured condition, at least the
thermoplastic material is phase separated from the base resin and
preferably phase separates into aggregate domains from the base resin,
each aggregate domain having an island-like morphology. The cure
morphology evolves (i) during the later stages of a ramp to dwell
temperature or (ii) after a ramp to dwell has been completed during the
cure cycle.

[0038] The amount of thermoplastic material in the formulation is below
approximately 30% net weight of the modified resin system and preferably
below approximately 7% net weight of the modified resin system.

[0039] The formulation may include an amount of particles and the amount
of thermoplastic material combined in a 1 to 0.56 ratio.

[0040] When the formulation is in a cured condition, the thermoplastic
material is phase separated from the base resin and preferably, the
thermoplastic material phase separates into aggregate domains from the
base resin, each aggregate domain having an island-like morphology.

[0041] Further embodiments of the present invention include a
manufacturing process, comprising preparing a preform, laying the preform
within a mold, heating the mold to a predetermined temperature and
injecting a resin wherein the resin is a modified resin, the modified
resin system comprising a combination of: (i) at least one base resin;
(ii) a curing agent; (iii) an amount of particles within a predetermined
range in a carrier resin; and (iv) an amount of thermoplastic material
within a predetermined range wherein the amount of thermoplastic material
of the modified resin is less than 30% net weight of the total weight of
the modified resin system.

[0042] The above manufacturing process may further modified wherein the
predetermined temperature of the mold is between 90.degree. C. and
120.degree. C. or more preferably the predetermined temperature of the
mold is 110.degree. C.

[0043] The manufacturing process may be practiced by ramping a temperature
of the mold to 180.degree. C. at a rate of up to 5.degree. C. per minute
or more preferably at a rate of 2.degree. C. per minute.

[0044] Furthermore, when the mold reaches 180.degree. C., the temperature
may be held about 120 minutes.

[0045] The manufacturing process may be practiced wherein the preform is a
plurality of layers of fiber-based fabric. The fiber-based fabric may
have a structure comprising one of woven fabrics, multi-warp knitted
fabrics, non-crimp fabrics, unidirectional fabrics, braided socks and
fabrics, narrow fabrics and tapes or fully-fashioned knit fabrics. The
fiber-based fabric may utilize reinforcing fibers of a material selected
from the group consisting of organic polymer, inorganic polymer, carbon,
glass, inorganic oxide, carbide, ceramic, metal or a combination thereof.

[0046] Furthermore, the manufacturing process is preferably practiced
where the preform is sealed within the mold by at least a vacuum bag.

[0047] Representative formulations according to embodiments of the
invention were prepared according to the following general Example:

Example 1

[0048] A base resin having di-, tri- and tetra-epoxies, a quantity of
amine curing agent and quantities of 5003P thermoplastic and CSR
particles (i.e. MX411) were combined. The combination (100 grams) was
transferred into steel molds which were then placed in a fan oven
preheated to 100.degree. C. (ramp to 180.degree. C. at 1.degree. C. per
minute, dwell for 2 hrs ramp to 25.degree. C. at 2.degree. C. per
minute). Samples (prepared from the cured modified resin plaque) were
prepared according to the relevant ASTM standard for the desired test.

Example A

Effect of Thermoplastic and CSR Concentrations on Resin Toughness

[0049] Experiments were conducted to quantify the effect of thermoplastic
(i.e., 5003P) in the absence of core-shell particles (i.e. MX411) (and
vice versa, i.e., core-shell particles) as toughening agents, thereby
providing a baseline for the toughening mechanism in the formulation
according to embodiments of the invention. The viscosity (.eta.) in the
base resin system (containing no CSR particles) was observed to increase
as the percentage loading of thermoplastic was increased, but to be
independent of CSR concentration. The toughness (G.sub.1C) of the systems
was found to increase with both increasing thermoplastic and CSR
concentration. It can be appreciated by one of ordinary skill in the art
that the use of CSR particles to achieve a high resin G.sub.1C does not
often translate into a high level of composite toughness performance. Due
to the combination of thermoplastic and CSR according to embodiments of
the invention, the toughness (G.sub.1C) versus viscosity behavior of the
formulation is closer to that of a CSR toughened material than that of a
thermoplastic toughened material (see FIG. 2).

Example B

Comparison of the Variation of CSR Particles to Thermoplastic Loading

[0050] Experiments were conducted to quantify the effect of CSR particle
(i.e., MX411) and thermoplastic (i.e., 5003P) loading in the base resin
(see FIG. 2). The viscosity (.eta.) of the material was found to increase
with thermoplastic and CSR content. The systems studied displayed
viscosity minima onsets which were found to vary with increasing
percentage masses of thermoplastic and CSR. The toughness (G.sub.1C)
behavior of the cured materials was found to follow an approximately
linear relationship with the viscosity of the system. Increasing the
percentage of thermoplastic and CSR (maintaining the 1 to 0.56 ratio) was
shown to give an unexpectedly high increase in the fracture toughness of
the neat resin when compared to the equivalent thermoplastic loading. The
morphology in the cured materials was shown to follow a similar fashion
to that expected from samples containing equivalent loadings of analogous
thermoplastic (see FIG. 3A, 3B).

Example C

Comparison of the Variation of CSR Particles to Thermoplastic Ratio

[0051] Experiments were conducted to quantify the effect of varying the
ratio of CSR particles to thermoplastic (see FIG. 3B). The toughness
(G.sub.1C) behavior of the neat resins was shown to follow a simple
linear relationship as established for other formulations. Additionally,
the presence of CSR and thermoplastic domains in the bulk resin phase was
shown to result in high G.sub.1C values for the neat resin. The size of
the proposed thermoplastic domains in the cured material was found to
increase with CSR content.

[0053] Experiments were conducted to compare different CSR particles
having different core chemistries. In this example, "A" is
poly(styrene-butadiene-styrene) or SBS core and "B" is polybutadiene
core. There was a negligible viscosity (.eta.) increase with systems
incorporating polybutadiene chemistry (CSR A) relative to systems
incorporating SBS core chemistry (CSR B).

[0054] In order to develop a formulation suitable for LRI, prepreg and
like applications while also resulting in appropriately toughened
laminate structures, the modified resin systems were targeted to remain
within a threshold limit of an average viscosity within a temperature
range while maintaining a high level of toughness (G.sub.1C). It was
discovered that formulations according to embodiments of the invention
complied with a threshold average viscosity of less than five (5) P with
a net weight of thermoplastic material of less than 30%, more narrowly
less than 7%, combined with an amount of CSR particles in a 1 to 0.56
ratio of thermoplastic to CSR particles, which resultant combined
characteristics rendered the modified resin system suitable for LRI
applications. The viscosity of less than (5) P was discovered to be
achievable at a temperature of less than 180.degree. C., more narrowly
between 80.degree. C. and 130.degree. C.

[0055] According to some embodiments, the thermoplastic material is
between about 0.1% and 7% net weight of the modified resin system and the
amount CSR particles is between about 0.1% and 10% net weight of the
modified resin system while maintaining a 1 to 0.56 ratio of
thermoplastic to CSR particles. In one embodiment, the thermoplastic
material is about 3.4% net weight of the modified resin system and the
amount CSR particles is about 1.9% net weight of the modified resin
system while maintaining a 1 to 0.56 ratio of thermoplastic to CSR
particles. It was discovered that the main contribution to achieving the
threshold viscosity was, among other factors, attributable to the
thermoplastic.

[0056] Representative formulations according to embodiments of the
invention are illustrated in the following Table 1:

[0058] Microcrack resistance is the ability of a material to resist
formation of small, numerous cracks upon induced stress and strain in the
material which instigates localized damage events that eventually weaken
and compromise the composite article. Microcrack resistance is typically
evaluated using multiple, simulated strain cycles. Samples are withdrawn
for microscopic analysis during the cycle phase and cracks are readily
identifiable after penetrative staining. During experiments, cured
modified resin samples showed no microcracks after 400 thermal cycles
(-53.degree. C. to 90.degree. C.) in one experiment and no microcracks
after 2000 thermal cycles in another experiment.

Example 2

[0059] Modified resin systems and unmodified or partially modified resin
systems were prepared and compared to study crack pinning, ductile
tearing and cavitation behavior of the systems expressed in fracture
toughness (K.sub.1C) values. The following systems were prepared: (i) a
modified resin system having thermoplastic and CSR particles (Formulation
4); (ii) a partially modified resin system having thermoplastic material
(Formulation 5); (iii) a partially modified resin system having CSR
particles (Formulation 6); and (iv) an unmodified resin system
(Formulation 7). Examination of the fracture surface of Formulation 4
illustrated multiple fracture toughness mechanisms at work. The
thermoplastic domains (i.e., 5003P) displayed ductile tearing and crack
pinning behaviors while the CSR particle domains (i.e., MX411) exhibited
features indicative of a cavitation mechanism (see FIG. 4). On the other
hand, examination of the fracture surface of the other Formulations 5, 6,
7 exhibited none or only partial similar damage resistance as that found
with respect to Formulation 4. Additionally the combination of a low
concentration of thermoplastic appeared to facilitate a more homogenous
dispersion of CSR particles than in Formulation 6. The following Table 2
summarizes these findings:

[0060] A numerical evaluation of the fracture toughness (K.sub.1C)
behavior demonstrated that Formulations 5, 6, 7 were relatively
indistinguishable from each other within the experimental parameters as
described above as compared to Formulation 4 (see FIG. 5). The K1C study
highlights the symbiotic relationship of thermoplastic material and CSR
particle toughening mechanisms within modified resin systems according to
embodiments of the invention. This was supported by an SEM investigation
which indicated that in the case of the proposed invention, the degree of
ductile failure was observed to be lower than that witnessed in
formulation 5. Additionally the degree of debonding between the
thermoplastic domains in the proposed invention was found to be
significantly less than that witnessed in Formulation 5 (ductile failure
and debonding of thermoplastic regions, FIGS. 5 and 6) and it was also
shown that the CSR domains Formulation 4 exhibited a cavitation-driven
toughening mechanism as opposed to the tear out mechanism witnessed in
Formulation 6 (FIG. 6).

[0061] Morphology Study

[0062] Evolution of Morphology as a Function of Temperature.

[0063] An investigation was conducted to elucidate the onset point of
morphology formation in the modified resin system, as embodied by the
proposed invention and prepared according to Example 1, during a cure
cycle. During this investigation, the morphology of the modified resin
system was determined to generally consist of a phase separation, more
particularly, an "island-like" morphology, of the thermoplastic and/or
CSR particles from the base resin. The "island-like" morphology is
generally a result of a thermally driven phase separation of the
thermoplastic from the base resin into discrete domains of
thermoplastic-rich material identified by a clearly defined border with
the cured or partially cured base resin when the modified resin system is
in a cured or partially cured condition. This morphology was shown to
evolve over a sixty (60) minute time interval during ramp-up temperature
followed by a constant temperature during a cure cycle. At zero (0)
minutes, between 80.degree. C. and 160.degree. C., the modified resin
components (thermoplastic, CSR particles and epoxy resin(s)) were shown
to be in a substantially uniform, dispersed phase. Between zero (0)
minutes and ten (10) minutes, between 170.degree. C. and 180.degree. C.,
thermally nucleated "seeds" began to evolve followed by development of
these seeds. Between ten (10) minutes and sixty (60) minutes, with the
temperature held constant at 180.degree. C., thermoplastic domains began
to evolve. At about sixty (60) minutes, the morphology of the
thermoplastic domains was seen to be substantially or completely evolved
(see FIG. 6). This unique processing factor, i.e., the controlled and
constant morphology evolution developed during a time period and at a
critical temperature (in this case, at about 180.degree. C.),
advantageously avoids flow and filtration issues which would otherwise
arise from having additive particles of the same size as the CSR
particles in conventional formulations. Through chemical modification of
the curing agent and the associated control of the resin vitrification
point it is expected that the morphology discovered by applicants will
develop at temperatures of less than 180.degree. C.

[0065] The morphology of the cured modified resin system (including the
development of thermoplastic domains) was determined to be generally
dependent upon the relative concentrations of CSR particles and
thermoplastic and, therefore, directly controllable.

[0066] Generalized Morphology.

[0067] An investigation was conducted to further elucidate the morphology
of the modified resin system. The investigation was performed by taking
images of the cured resin using a scanned electron microscope (SEM) and a
transmission electron microscopy (TEM). The results of the TEM and SEM
investigations suggest that the thermoplastic domains form via a phase
separation of thermoplastic from the base resin during the cure of the
resin while the CSR particles remain located within the base resin and
are not drawn into the thermoplastic domains (see FIGS. 3A, 3B, 4). The
TEM evidence is supported by SEM evidence indicating the presence of
growth rings within the thermoplastic morphology (see FIGS. 7A, 8, 9) and
also a combined optical microscopy/differential scanning calorimetry
(DSC) study demonstrating the onset of morphology growth at the point
where the resin begins to vitrify (see FIG. 6).

[0068] FIG. 3A illustrates a schematic of the generalized morphology of
the modified resin system according to embodiments of the invention as
discovered by the inventors. As shown in FIGS. 3A-3B (see also FIGS. 4,
6-9) addition of thermoplastic within a predetermined range (as well as
CSR particles within a predetermined range) to a base resin (having one
or more resins) resulted in a thermally-induced phase separation of the
thermoplastic material from the base resin during the cure cycle of the
modified resin system. Furthermore, the CSR particles were observed to
partially, substantially or completely remain within the base resin and
were not therefore experimentally determined to be incorporated into the
thermoplastic material domains.

[0069] In addition to being advantageous with respect to processing (see
Evolution of morphology above), the morphology of the modified resin
system discovered by the inventors is believed to contribute to the
combination of high Compressive After Impact Strength (CSAI), K.sub.1C
toughness (G.sub.1C), and microcrack resistance required for composite
articles exposed to damage caused by environmental conditions and/or
events while simultaneously allowing for a wide processing window during
the fabrication process. It is anticipated that any thermoplastic
exhibiting phase separation morphology, more particularly, an
"island-like" morphology, combined with a suitable nanoscale particle
(i.e., CSR or hollow particle) would be appropriate for formulating
modified resin systems according to embodiments of the invention.

Processing Methods Using LRI

[0070] FIG. 10 illustrates a representative LRI approach (e.g., Resin
Infusion in Flexible Tooling (RIFT)) having a fabric preform thereon. As
shown, the system includes a single-sided tool (i.e., mold) 1002 with a
fiber preform 1004 laid thereon. A peel-ply layer 1006 may be applied to
a surface of preform 1004. A vacuum bag 1008 having a breather 1010
therein seals preform 1004 therein creating a "cavity", or area in which
preform 1004 resides. Before preform 1004 is laid on tool 1002, a release
agent or gel coat 1012 may be applied to a surface of tool 1002 and/or to
a surface of vacuum bag 1008. At one end, the "cavity" is connected to a
resin inlet 1014 via a resin transfer line (not shown). At another end,
or at the same end, the "cavity" is connected to a vacuum system (not
shown) via a vacuum evacuation line 1016. Once preform 1004 is positioned
within tool 1002 and vacuum is applied, a liquid resin 1018 may be
infused into the "cavity" at ambient pressure, a predetermined pressure
or a gradient pressure. Liquid resin 1018 may be infused at ambient
temperature, a predetermined temperature or a temperature gradient.

[0071] According to embodiments of the invention, modified resin systems
(as described previously) may be applied to preforms constructed from one
or more layers of engineered textiles to manufacture composite articles
using LRI processing techniques and tools (such as that represented in
FIG. 10). The engineered textiles may include, but are not limited to,
woven fabrics, multi-warp knitted fabrics, non-crimp fabrics,
unidirectional fabrics, braided socks and fabrics, narrow fabrics and
tapes and fully-fashioned knit fabrics. These fabric materials are
typically formed of fiber glass, carbon fiber, aramid fibers,
polyethylene fibers or mixtures thereof. When the preform is subjected to
LRI, LRI-derived laminates are produced.

[0072] Representative laminate test samples having the modified resin
system according to embodiments of the invention infused therein were
prepared according to the following general example.

Example 3

[0073] Initial lay-ups of non-crimped fiber (NCF) fabric (8 ply layup)
were prepared for RIFT (see FIG. 10) to produce laminate test samples. In
this embodiment, the fabric was of a carbon material. Laminate test
samples were also prepared using a closed mold RTM press set at 25
cm.sup.3/minute flow rate and an eight (8) millimeter (mm) inlet. In both
cases, the resin pot was held constant at 100.degree. C. and the tool was
held constant at 110.degree. C. for infiltration of the resin prior to
commencing a 2.degree. C. per minute ramp towards 180.degree. C.,
dwelling for two (2) hours before ramping down at 2.degree. C. per minute
to room temperature. Generally, the tool may be heated to a temperature
of between 130.degree. C. and 180.degree. C. at a rate of less than
10.degree. C. per minute.

[0074] Various tests were performed on the laminate samples in order to
determine compliance with threshold mechanical performance parameters.
Key mechanical properties evaluated included storage modulus-derived
glass transition temperature, elastic modulus, Compressive Strength After
Impact (CSAI) and open hole compression (OHC) strength (wet and dry).

Laminate Mechanical Properties

[0075] Dynamic Mechanical Thermal Analysis (DMTA) was performed to
determine the glass transition temperature (T.sub.g) of laminate test
samples in accordance with known methods. Glass transition temperature is
indicative of a laminate article to carry mechanical load. Suitable
ranges are between 130.degree. C. and 210.degree. C. for T.sub.g (dry)
and between 110.degree. C. and 170.degree. C. for T.sub.g (wet). Modified
resin systems according to embodiments of the invention were found to
have a T.sub.g (dry) between 140.degree. C. and 190.degree. C. and
between 140.degree. C. and 160.degree. C. (wet).

[0076] In-plane shear modulus was measured for laminate test samples
according to known methods. In-plane shear modulus was determined to be
between 3.5 GPa and 4.5 GPa (dry/RT) and between 3.0 GPa and 4.0 GPa
(hot/wet).

[0077] Damage Resistance/Tolerance FRP Materials.

[0078] Damage resistance is the ability of the composite article to resist
damage after a force event which may cause delamination and weakening of
the composite article and is a critical parameter for in-service behavior
in high performance applications. Damage resistance can be measured
through dent depth analysis or C-scan damage area analysis of impacted
composite samples. Damage tolerance can be measured by a Compressive
Strength After Impact (CSAI) test.

[0079] Laminate test samples prepared using modified resin systems
according to embodiments of the invention exhibited reduced dent depths
when compared to prior art laminates. In one experiment, laminate test
samples were found to have an average dent depth of between 0.6 mm and
0.8 mm following an impact event. These values represent about a 10%
decrease in dent depth when compared to prior art laminates. In another
experiment, laminate test samples were found to have CSAI values between
about 220 and 270 Mega-Pascals (MPa) in a plain weave textile (see FIG.
11) and between about 200 and 225 MPa in a non-crimp fiber textile which
indicate a high tolerance to damage after an impact event. OHC values
were experimentally determined to be between 280 MPa to 320 MPa (dry) and
between 220 MPa and 260 MPa (hot/wet).

[0080] The unexpected stable and low average viscosity (i.e., less than 5
P) of modified resin systems with a suitable toughness according to
embodiments of the invention combined with the high microcrack resistance
exhibited by resultant LRI-derived laminate articles renders it suitable
for the manufacture of complex structures in a range of different
industries including the aerospace, transport, electronics, building and
leisure industries. Specific to the aerospace industry, the modified
resin systems may be used to construct components including, but not
limited to, frame and stringer-type components for twin aisle derivatives
and single aisle replacement programs, fuselage shell components,
integrated flight control components for replacement programs, wing box
structures and rotorblade systems for rotorcraft. Additionally, the
modified resin systems may be used in the manufacture of composite for
complex textile systems.

[0081] While certain exemplary embodiments have been described and shown
in the accompanying drawings, it is to be understood that such
embodiments are merely illustrative of and not restrictive on the broad
invention, and that this invention is not to be limited to the specific
constructions and arrangements shown and described, since various other
modifications may occur to those ordinarily skilled in the art.